1. Field of the Invention
The present invention is related to the field of tappets for internal combustion engines. More specifically, the invention relates to such tappet assemblies having a ceramic wear pad.
2. Description of Related Art
In internal combustion engines, camshafts with cam lobes are generally used to actuate various engine components such as valves and other related valve train components as well as fuel injectors. In particular, in engine valve trains, a tappet is generally provided to contact the cam lobe and to follow the cam lobe profile as the camshaft rotates. In this manner, the tappet is displaced and exerts a force to displace an engine component mechanically connected to the tappet such as a push rod, rocker arm or a valve.
Such valve train components are subject to high forces and stresses. In particular, the contact interface between the cam lobes and the tappets are subject to very high sliding contact stresses as the camshafts arc rotated and the cam lobes slide across the surface of the tappets to thereby displace the tappets. To ensure durability of the cam lobes and the tappets under these adverse operating conditions, these engine components are typically made of metals such as iron or steel or other strong materials that are durable enough for such applications. However, with increased engine performance requirements, the speed of rotation of the camshafts and the forces excited between the cam lobes and the tappets have increased correspondingly. As a result, the iron or steels typically used for the tappets have been found to be insufficient in allowing the increased engine performance. Moreover, the consumers of such engines have come to expect increased durability and minimal repair of these engines as well.
Consequently, various tappet assemblies with wear resistant ceramic pads have been created to allow increased engine performance and to meet the increased durability requirements. For instance, U.S. Pat. No. 5,060,607 to Taniguchi discloses a tappet assembly with a friction-resistant ceramic plate fixedly attached to an end of a tappet body. The reference discloses that the ceramic plate is made of silicon nitride or zirconia as well as other ceramics. The reference further discloses an alternative embodiment in which the ceramic plate is provided on a metallic plate which may be brazed on to the tappet body. It has been found that the ceramic plate made of such materials as silicon nitride or zirconia reduced friction between the cam lobe and the tappet assembly thereby reducing wear on both of these components and potentially provided a solution for meeting the increased performance and durability requirements.
However, concern exists that over a period of use, the ceramic plate as disclosed in Taniguchi would tend to separate and eventually slide off from the tappet body as the cam lobe exerts a sliding force on the ceramic plate""s surface. Consequently, the tappet assembly with the ceramic plate as disclosed in Taniguchi has been found to be insufficient to allow increased engine performance and insufficient to meet the increased durability requirements.
To improve upon the tappet assembly disclosed in Taniguchi, U.S. Pat. No. 5,372,099 to Matsunuma et al. and U.S. Pat. No. 5,445,119 to Regueiro each disclose a ceramic shim for minimizing abrasion of a cam and a tappet which includes a recessed pocket formed by a lip of the tappet""s peripheral wall for partially receiving the ceramic shim in the recessed pocket. The Matsunuma reference discloses a clearance between the ceramic shim and the tappet""s peripheral wall. In contrast, the Regueiro reference discloses a dual-diameter shim with a small and large diameter cylindrical portions where only the small diameter cylindrical portion is received in the recess pocket of the tappet. It has been found that because of the recessed pocket formed by the lip of the tappets"" peripheral wall, the ceramic shims were prevented from sliding off from the respective tappet body as the cam lobe exerts a sliding force on the ceramic plate""s surface. Thus, the primary disadvantage of the tappet assembly disclosed in Taniguchi is eliminated.
However, the present applicants have found that the tappet assemblies with the ceramic shims disclosed in Matsunuma and Regueiro discussed above present unique problems of their own. In particular, it has been found that over a period of use, the ceramic shims may tend to rotate within the recessed pocket. This poses a significant durability and reliability concerns since the free rotation of the ceramic shim substantially reduces or even eliminates rotation of the tappet body within its mating bore in the cylinder block, this rotation of the tappet body being necessary for maximizing wear and scuffing resistance at that interface (i.e. the interface between the tappet body and the mating bore). More significantly, such free rotation within the recessed pocket can cause excessive wear on the lip of the tappet""s peripheral wall and eventually cause the ceramic shim to slide off the tappet body which was the problem encountered in the tappet assembly disclosed in Taniguchi. Moreover, this tendency for rotation of the ceramic shim relative to and within the recessed pocket has been found to be exacerbated in diesel engine applications where soot particles accumulate on the contact surfaces of the cam lobe and the ceramic shim thereby increasing the coefficient of friction between these components. Thus, like the tappet assembly disclosed in Taniguchi, the tappet assemblies with the ceramic shims disclosed in Matsunuma and Regueiro have also been found to be insufficient to allow increased engine performance and insufficient to meet the increased durability requirements as well.
Therefore, there exists an unfulfilled need for a tappet assembly with a ceramic wear pad which will allow increased engine performance. There also exists an unfulfilled need for such a tappet with a ceramic wear pad which will increase durability of the tappet assembly.
In view of the foregoing, it is an object of the present invention to provide an improved tappet assembly with a ceramic wear pad which will allow increased engine performance.
A second object of the present invention is to provide an improved tappet assembly with a ceramic wear pad which will provide increased durability of the tappet assembly.
Yet another object of the present invention is to provide a tappet assembly with a ceramic wear pad which will prevent the ceramic wear pad from sliding off from the tappet body.
Still further, it is an object of the present invention to provide a tappet assembly with a ceramic wear pad which will prevent the ceramic wear pad from rotating relative to the tappet body.
In accordance with preferred embodiments of the present invention, these objects are obtained by an improved tappet assembly for an internal combustion engine, the tappet assembly having a first end mechanically connected to an engine component and second end adapted to contact a cam lobe of a camshaft to displace the engine component, where the tappet assembly includes a tappet body having a recessed pocket positioned at one end thereof, the recessed pocket being defined by an extended peripheral lip of the tappet body, a ceramic wear pad at least partially received within the recessed pocket, the ceramic wear pad being adapted to contact the cam lobe, and a rotation prevention means for preventing the ceramic wear pad from rotating relative to the recessed pocket. The ceramic wear pad has a top surface for contacting the cam lobe, a bottom surface which is received in the recessed pocket, and a peripheral edge surface thereinbetween.
In accordance with one embodiment of the present invention, the rotation prevention means includes at least one inwardly angled portion of the peripheral lip, the at least one inwardly angled portion being swaged radially inwardly toward an axial center of the ceramic wear pad to engage the ceramic wear pad. The ceramic wear pad has at least one engagement surface on the peripheral edge surface and at least one inwardly angled portion engages the at least one engagement surface. Preferably, the at least one engagement surface is a plurality of engagement surfaces distributed along the peripheral edge surface and the at least one inwardly angled portion is a plurality of inwardly angled portions that engages the plurality of engagement surfaces.
In accordance with another embodiment of the present invention, the peripheral edge surface of the tappet assembly includes a beveled edge so that the top surface of the ceramic wear pad has a smaller diametric dimension than the bottom surface. The plurality of engagement surfaces are equidistantly distributed on the beveled edge. Thus, in this embodiment, the plurality of inwardly angled portions are swaged radially inwardly toward the axial center of said ceramic wear pad so as to engage the plurality of engagement surfaces on the beveled edge.
In accordance with yet another embodiment of the present invention, the rotation prevention means includes at least one substantially flat surface provided on the peripheral edge surface of the ceramic wear pad. A set screw threaded through a hole on the extended peripheral lip is provided to fixably engage at least one substantially flat surface on the peripheral edge surface of the ceramic wear pad. Of course, such provisions of a set screw may also be provided in other embodiments of the present invention as well. In another alternative embodiment, a weldment may be used instead or in conjunction with the set screw.
In still another embodiment of the present invention, the tappet assembly may further comprise an intermediary layer disposed in the recessed pocket between the bottom surface of the ceramic wear pad and a base surface of the recessed pocket. In one embodiment, this intermediary layer call be disposed solely between the ceramic wear pad and the tappet body. In another embodiment, the intermediary layer can extend around the sides of the ceramic wear pad to facilitate centering the ceramic wear pad within the recessed pocket. The intermediary layer may be an adhesive layer for adhering the bottom surface of the ceramic wear pad to the base surface of the recessed pocket. The adhesive layer may be made of epoxy or be formed of a dry polymer film which is adapted to be melted and resolidified. Alternatively, the intermediary layer may be a compliant layer made of a material which is more compliant than the tappet body and the ceramic wear pad. In this regard, the compliant layer may be a metallic sheet, a metallic shim, a wire mesh made of aluminum, copper or other ductile material such as a metal or an alloy, or be made of a polymer.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention when viewed in conjunction with the accompanying drawings.